1. Field of the Invention
The present invention relates to an image reading apparatus for reading an image of an original, such as an image scanner. The reading apparatus could be an image reading unit mounted on an image forming apparatus, such as a digital copier or a facsimile machine, or a stand-alone image reading apparatus.
2. Description of the Related Art
A typical image reading apparatus includes an image reading means for scanning the printed side of an original in a main scanning direction with a light source turned on and performing a photoelectric conversion of a reflected light from the printed side on a line-by-line basis with a photoelectric conversion element to read as an analog image signal and an A/D conversion means for converting the analog image signal read by the image reading means into a digital image signal and outputting the digital image signal.
Moreover, the image reading apparatus detects (generates), as a black level, a gray level of a digital image signal output from the A/D conversion means and holds the black level. The black level is detected based on a gray level of a digital image signal output from the A/D conversion means in a state where no light enters the photoelectric conversion element as a reference level. When reading an image of an original, the image reading apparatus performs an offset correction, for eliminating an offset, by subtracting the black level from a gray level of a digital image signal output from the A/D conversion means. A CCD (Charge Coupled Device) can be used in a photoelectric conversion element. The CCD has one output buffer corresponding to a group of pixels so that the black level varies per output buffer. Therefore, when a CCD is employed in a photoelectric conversion element, a common black level is subtracted from the value of each pixel in the group of pixels corresponding to an output buffer.
In general, a photoelectric conversion element in which a CCD is employed has an effective image portion and an optical black (hereinafter, referred to as “OPB”) portion. The OPB is a portion on which the light does not fall. A black level, i.e., a gray level of a portion of a digital image signal corresponding to the OPB portion (hereinafter, referred to as simply “the black level of the OPB portion”) is detected and held (for example, see Japanese Patent Application Laid-open No. 2007-158663).
On the other hand, a CMOS (Complementary Metal Oxide Semiconductor) can be employed in a photoelectric conversion element instead of a CCD. A CMOS has one output buffer for every pixel so that the black level varies per pixel. As a result, when a CMOS is employed in a photoelectric conversion element, a normal offset correction cannot be performed by subtracting a value of the black level of the OPB portion with respect to all the pixels as in the case of the CCD. Therefore, in general, pixel-by-pixel black levels are detected with a light source turned off and held in advance before an image of an original is read, and an offset correction is performed by subtracting the respective black levels on a pixel-by-pixel basis when an image of the original is read.
Image reading apparatuses is which a contact image sensor (CIS) is employed as a reading system are known in the art. On the other hand, CISs in which a plurality of sensor IC chips, a photoelectric conversion element that employs a CMOS, are arranged, for example, in a zigzag manner shown in FIG. 8 (i.e., aligned along the main scanning direction with an overlap between adjacent sensors), and combined with a 1×-magnification optical system are known in the art.
An image reading apparatus in which a CIS is employed detects a black level on a pixel-by-pixel basis and holds the detected black level, and subtracts the black level on a pixel-by-pixel basis when reading an image of an original.
However, in an image reading apparatus in which a CIS is employed, differences in black level among sensor IC chips are caused by temperature characteristics of the sensor IC chips and variation factors with respect to each of the sensor IC chips, such as a circuit path. The differences in black level among sensor IC chips can not be corrected. The reason is that, after pixel-by-pixel black levels have been generated, the black levels vary when an image of an original is read, and thus the differences in black level cannot be eliminated by the pixel-by-pixel black-level correction (offset correction) performed before an image of the original is read.